The present invention pertains generally to liquid-crystal-on-silicon (LCoS) microdisplay systems and, more particularly, to a high contrast microdisplay system using a vertically aligned nematic reflective cell design.
Microdisplays are the most recent addition to the family of flat-panel displays. Microdisplays are based on a number of different techniques to generate modular light using microfabrication technologies to produce a rectangular array of pixels on a semiconductor back plane. Examples of microdisplays include liquid crystal displays, field emission displays, and digital micro-mirror displays.
Liquid crystal displays (LCD) using a single polarizer plate (herein referred to as single polarizer plate method) provide high resolution and high contrast. Many LCD""s utilize a quarter-wave plate to produce high contrast.
Japanese Laid-Open Patent Application No. 55-48733/1980 (Tokukaisho 55-48733/1980) discloses a reflective TN-method (45xc2x0 twisted) LCD having one polarizer plate and a quarter-wave plate. By using a 45xc2x0 twisted LC layer and controlling the electric field applied across the LC layer, the device displays black and white by effecting two states in which the polarization plane of the incident linearly polarized light on the quarter-wave plate is parallel and twisted by 45xc2x0 with respect to the optic axis of the quarter-wave plate. The LCD display includes a polarizer, a 45xc2x0 twisted LC layer, a quarter-wave plate, and a reflective plate arranged in sequence from the light incident side.
U.S. Pat. No. 4,701,028 (Clerc et al.) discloses a reflective-type LCD device including a combination of a single polarizer plate, a quarter-wave plate, and a vertically aligned LC cell. Japanese Laid-Open Patent Application No. 6-337421/1994 (Tokukaihei 6-337421/1994) discloses a reflective-type LCD device including a combination of a single polarizer plate, a quarter-wave plate, and a bend vertically aligned LC cell. A New Reflective Display with High Multiplexibility and Gray Scale Capability (Euro Display ""96, page 464) also discloses a reflective-type LCD device including a combination of a single polarizer plate, a quarter-wave plate, and a vertically aligned LC cell. SID 96 Digest (page 763) discloses an example of an application to a reflective projection of a display mode in which chiral-dopant-containing LC having negative dielectric anisotropy is sandwiched between upper and lower substrates with vertical alignment.
A polarizer plate, in combination with the LCD device cell on the incident side, allows only the incident light and outgoing light polarized linearly in a certain direction to pass therethrough, and blocks light that is polarized linearly in all the other directions. If no electric field is applied to the LC layer, the incident light having passed through the polarizer plate is converted into circularly polarized light by an optical retardation compensation plate such as a xcex/4 plate (quarter-wave plate), enters the LC layer, passes through the vertically or substantially vertically aligned LC layer, and without any further conversion or change, reaches a reflective plate. The light having reached the reflective plate is converted into circularly polarized light of reverse rotation by the reflective plate, passes through the LC layer, the xcex/4 plate and other components in reverse order from the incident light, and is converted to light that is linearly polarized vertical to the linearly polarized incident light. The dark state is thus effected.
In addition, if the LC inclines or tilts upon application of an electric field across the LC layer to realize a phase difference under certain conditions, the incident circularly polarized light (having passed through the polarizer plate and xcex/4 plate) is converted into linearly polarized light, becomes linearly polarized at the reflective plate, resulting in out-going linearly polarized light having the same polarization direction as the linearly polarized light at the polarizer plate, which is the incident light for the display device having passed through the polarizer plate. The bright state is thus effected.
There are some problems with the vertical alignment LCD device described in U.S. Pat. No. 4,701,028, U.S. Pat. No. 4,492,432, Japanese Laid-Open Patent Application No. 6-337421/1994, and A New Reflective Display with High Multiplexibility and Gray Scale Capability (Euro Display ""96). Since the directions of the vertical alignment, especially those of the tilted vertical alignment, are parallel between the upper and lower substrates, the LC inclines or tilts in one direction and the viewing angle dependence of the image on the display surface is extremely large. Also, since the reflectance has a large wavelength dependence, undesirable coloring occurs.
Many LCD devices adopt a planar alignment mode where LC molecules are aligned parallel to the substrate when no voltage is applied. In a vertical alignment mode, LC molecules are aligned vertically relative to the substrate when no voltage is applied. When a LCD device operates in a so-called normally black mode, in which dark display is effected using the vertical alignment when no voltage is applied, a darker and more achromatic black display can be effected in comparison to the planar alignment mode, and therefore the display contrast is improved. Since the LC layer does not cause birefringence with light transmitted in the normal direction of the LC layer through the LC layer when no voltage is applied across the LC layer, the normally black mode has the characteristic that the required accuracy in the LC layer thickness (LC cell gap) is less.
In the vertical alignment mode, the LC layer thickness can be larger than the conventional planar alignment mode, there can be wider variations in the cell gap, and improved black display is possible. The primary factor limiting high contrast in LCD""s is the amount of light which leaks through the display in the dark state. If vertical alignment normally black mode is used, there is little negative effect from light leaking as a result of the use of spacers to maintain the cell gap of the LC layer. For these reasons, vertical alignment is a preferred alignment for high contrast displays.
However, a uniform alignment state is difficult to achieve with the vertical alignment mode. It is also difficult to effect stable alignment when an electric field is applied across the LC layer and the alignment of the LC layer molecules is deformed from the vertical direction.
Publications including Enclosure Electrode Method (Japanese Laid-Open Patent Application No. 7-64089/1995 [Tokukaihei 7-64089/1995]) and Multidomain Method (Nikkei Microdevice, January 1998 Issue, page 136) disclose a proposal to change the shape of the substrate by changing the direction of the electric field, or by use of an insulating structure within the pixel area of a display.
Methods to slightly incline (tilt or pre-tilt) the initial LC alignment with respect to the normal direction of the substrate by rubbing (Japanese Laid-Open Patent Application No. 62-299814/1987 [Tokukaisho 62-299814/1987]) or by photo-induced polymeric alignment (Control of Liquid Crystal Alignment Using an Optically Active Polymer Film, by Mr. Yasushi IIMURA, Tokyo University of Agriculture and Technology, First JLCS-ALCOM Joint Conference) are disclosed to obtain uniform alignment with respect to the vertical alignment over the entire panel when voltage is applied. These methods provide alternative solutions to the problem other than the creation of microdomains. Unfortunately, this pretilt angle has been shown to reduce the effectiveness of the black state. (H. Kurogane, K. Doi, T. Nishihata, A. Honma, M. Furuya, S. Nakagaki, I. Takanashi, Victor Company of Japan, SID Dig. Tech. Pap., 29, 33-36).
The enclosure electrode method and the multidomain method sandwich the upper and lower substrates LC layer in a precise and complex structure. This results in an increased number of manufacturing processes, and the need to make a domain having a period of repetition equal to the size of a pixel. The period of repetition of an insulating component or the structure of an enclosure electrode, which defines the period of repetition of the domain cannot be designed freely due to the need to effect stable alignment, and must be designed to fall in a certain range.
To achieve a stable vertical alignment, through the electrode structure or substrate structure, the rubbing and photo-induced alignment methods that are free from such problems are preferable. However, it would be most preferable to eliminate these pre-tilt alignment manufacturing steps altogether.
In an attempt to eliminate pre-tilt alignment techniques while taking advantage of the properties of vertically-aligned LCD""s, electric-field-aligned multi-domain perfectly homeotropic (PH) (also called super homeotropic (SH)) mode vertically aligned nematic (VAN) LCD""s have been developed. These cells have liquid crystal molecules that are oriented in a perfectly homeotropic direction (perpendicular to the cell surface) which is uniform from one cell surface to another. They allow for wide-angle viewing, high contrast, and fast response characteristics. In these modes, LC alignment is divided into four regions for each pixel. Cells in these modes require that the LC director be aligned at a 45xc2x0 angle from the transmission axes of linear polarizers. Any deviation from the ideal 45xc2x0 angle reduces brightness and creates a dead space or a dark region at the boundaries of the four domains. These dark regions cause a reduction of effective aperture ratio, thereby resulting in low light transmittance of the panels.
SID 00 Digest (page 902-905) disclosed a method for improving light efficiency for transmissive LCD""s. The method is aimed toward solving the problem noted above. Two xcex/4 retardation films oriented at 45xc2x0 with respect to the x-axis of linear polarizers were used in order to achieve circularly polarized light propagation through the LC cell. Because the light is circularly polarized, rather than plane polarized, the transmitted light intensity is irrespective of the in-plane orientation of the LC director. Therefore, because the intensity of transmitted light depends only on the optical retardation, the dark regions resulting from derivation of the angle of the LC director are eliminated.
One particular type of reflective liquid crystal (LC) technology, the liquid-crystal-on-silicon (LCoS) microdisplay is emerging as an attractive choice for a wide variety of applications. These applications include: multimedia front projectors, rear-projection computer monitors, rear-projection televisions, and near-to-the-eye (NTE) displays. The advantage of LCoS over other reflective type LC devices is that LCoS provides high performance, high-information-content microdisplays at significantly lower cost than competing technologies. The is due in part to the fact that the active matrix of driving circuitry for the pixels of the display is produced by conventional silicon transistor fabrication.
In addition, reflective-type direct view LCD""s offer a number of design advantages over transmissive-type LCD""s. A reflective LCD, since adopting a different display method from the transmissive LCD with no backlight, can save the power consumption for the light source. In reflective LCD""s, the space and weight for the backlight can be spared. In other words, the reflective LCD consumes less power and can obtain enough power from a smaller battery, being suitable for devices whose priority is light weight and thin size. If the reflective LCD is designed having the same size or weight as a transparent LCD, the reflective LCD can accommodate a larger battery, and therefore operate much longer than a transparent LCD.
Another feature of the reflective LCD can be found in the contrast characteristics of the display surface. The light emitting display, for example a Cathode Ray Tube (CRT), has a greatly decreased contrast ratio in the daylight. A transparent LCD, having undergone low reflection processing, also suffers from a largely decreased contrast ratio when the surrounding light, for example, direct sun light, is strong in comparison to the display light. A reflective LCD provides display light proportional to the amount of surrounding light, being especially suitable for displays used outdoors, such as portable information terminals, digital cameras, and portable video cameras.
In accordance one aspect of the present invention, a light valve for a microdisplay is provided which utilizes circularly polarized light and a homeotropic vertically aligned nematic mode reflective liquid crystal cell. In one embodiment, an on-axis design is disclosed in which a circular polarizer is used to circularly polarize non-polarized light from a light source. The circularly polarized light is then reflected by a non-polarizing beam splitter toward a reflective liquid crystal cell. After the light is reflected by the liquid crystal cell, a second circular polarizer is used to either block the light if the microdisplay is in such a state that it does not affect the light, or transmit the light if the microdisplay is in such a state that it retards the light by a quarter wave upon a single pass. In another aspect of the present invention, an off-axis light valve for a microdisplay is provided which does not utilize a beam splitter and in which light is incident on both circular polarizers and on the liquid crystal cell off-axis.
Also according to the present invention, there is provided a LCD device and method of producing a high contrast microdisplay with vertically aligned nematic mode reflective liquid crystal cell having no pre-tilt inducing mechanism. The use of circularly polarized light eliminates the need for exact domain size and shape requirements, and the need for specific director orientation. Therefore, fringe fields alone are sufficient to induce a director tilt, thereby eliminating the need for pre-tilt inducing or rubbing processes.
In another aspect of the present invention, a light valve utilizing a single circular polarizer to achieve ultra high contrast is provided for use in a high contrast direct view reflective liquid crystal display device.
Advantages of the present invention are: a reflective display with perfectly homeotropic alignment in vertically aligned nematic mode that achieves ultra high contrast; a method of producing a high contrast reflective display that can be manufactured without utilizing rubbing, ultraviolet, or other pretilt-inducing process; an MVA microdisplay without domain size and shape requirements; elimination of the negative effects of fringing fields on image quality of reflective microdisplays; a reflective display that is capable of use with a wide range of different wavelengths of light.
Still other advantages of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description, accompanying drawings and claims.